As a typical element between metallic elements and non-metallic elements, selenium is widely used in glass and dyeing industries. Selenium plays an irreplaceable role in optimizing performances of glass, preparing a pure red dye of high temperature resistance or the like. In material science, adding selenium into steel, copper and rubber, etc. in various proportions may significantly improve oxidation resistances, mechanical performances and abrasive resistances thereof. The most promising area for selenium may be photoelectric industry. Moreover, due to the unique characteristics of selenium, such as special electrical conductivity and excellent light absorption, selenium plays a vital role in the technical field of semiconductor, laser and electronic components, and these characteristics bring great values in use and demands for selenium.
At present, the recovery of commercial selenium is mainly concentrated in the field of hydrometallurgy. Moreover, selenium is mainly prepared by the processes for conventional selenium evaporation and reduction in copper anode slime, for leaching and reduction of a complex selenium-containing slag material, and for oxidizing leaching and reduction of a selenium-containing secondary material, etc.
However, in the current solution, there are two kinds of knotty problems in the selenium reduction process.
(1) In the selenium reduction process, it is easy to generate negatively bivalent selenium in the solution. Thus, the elemental selenium obtained by the reduction dissolves in the form of ion again, or generates an insoluble compound with heavy metals such as copper, leading to a direct decrease in the recovery rate. Besides, the subsequent treating processes prolong the flow path of the preparation process, and increase the difficulty of selenium recovery process.
(2) In an extraction and separation process of a selenium-containing material, selenium is often separated through oxidizing leaching, roasting or the like, by taking advantage of the characteristic that the selenium and its compounds are easily oxidized and decomposed. However, in the oxidation process, part of the selenium may be over oxidized to hexavalent selenium. The current reduction methods of hexavalent selenic acid or hexavalent selenate are shown as follows.
(i) Elemental selenium is obtained through reducing hexavalent selenium by boiling the latter with high concentrated hydrochloric acid. However, a large amount of acid fog will be generated in a reduction process, and an improper operation may also lead to the leakage of a harmful gas such as chlorine gas. As a result, the possibility of the practical application is low.
(ii) An adsorption-reduction process is carried out using ferrous iron. Ferrous iron may effectively reduce hexavalent selenium to elemental selenium, but the results of the current studies indicate that a reduction with ferrous iron is only suitable for treating a hexavalent selenium solution having an ultra-low concentration. Besides, the reaction rate is slow; and the reduction product is an iron-selenium complex compound, so that it still needs a selenium-iron separation to recover selenium separately, and thus the flow path of the process is complex.
(iii) An adsorption-precipitation process of hexavalent selenium is carried out using an aluminum salt compound. Its problems are similar to those in the ferrite reduction process, that is, its processing capacity is low, it is only suitable for treating the selenium solution having an ultra-low concentration, the reaction rate is slow; and the flow path of the process is complex, and the reduction product is an aluminum-selenium compound.
Hence, in practical industrial production, it is very difficult to effectively recover hexavalent selenium using the existing selenium production procedure, resulting in that hexavalent selenic acid or selenate can only enter a wastewater treatment process and that a lot of resources are wasted. It is because of the defects in the prior art, such as high requirement for reducing hexavalent selenium, long flow path of recovery, etc., that there is still absence of a technique for the recovery of hexavalent selenium with a high recovery rate, a short flow path of the process and a high industrial feasibility.